This invention relates to the field of flexible circuits. More particularly, this invention relates to the field of flexible circuits in which an area of the circuit is subjected to repeated flexing to accommodate the movement of a component to which the flexible circuit is connected. As used herein, the term "flexing" refers to cyclic bending of a predetermined portion of the flexible circuit.
The field of computer disc drives is one to which this invention is particularly pertinent, so that field will be discussed for purposes of illustration of the features, utility and advantages of this invention. However, it is to be understood that this invention is not limited to circuits for disc drives; rather, this invention is generally suitable for any application in which it is desired to facilitate flexing of a flexible circuit.
In computer disc drives the read/write head scans the disc, i.e., moves over the surface of the memory disc, to perform its read/write function. A flexible circuit connects the read/write head to the computer system for signal transmission between the disc and the computer. The flexible circuit flexes, generally in what is referred to as a "rolling" or "roll flexing" mode, to accommodate and permit linear movement of the read/write head in a radial direction with respect to the memory disc. A certain amount of energy is, of course, required to flex the flexible circuits. The flexing action of a flexible circuit can be viewed and analyzed as analogous to cyclic beam bending, and the amount of energy consumed to effect flexing is a direct function of the stiffness of the flexible circuit in the area where the bending occurs. As disc drive technology has evolved, disc diameter has gotten progressively smaller, going from 8" to 51/4 to 31/2, and now looking to about 2" for the next generation of disc drives. This has led to a reduction in the length (and generally the overall size) of the flexible circuit and a reduction in the section of the flexible circuit that is subjected to dynamic flexing, and hence an increase or concentration of stress in that flexing section. Also, for newer disc drive applications it appears that designers are favoring what is referred to as the "book binding" approach rather than the traditional "roll flexing" in the design of the bending mode of the flexible circuit. This "book binding" approach results in a more compact circuit design with a shorter segment of the circuit which is subjected to the flexing action, thus further concentrating the area of stress.
A further related factor to be considered is that as computer systems become smaller and/or portable (e.g., lap top), it becomes increasingly important to reduce the amount of energy required to flex the flexible circuit to conserve the battery.
Other factors to be considered are the mass of the flexible circuit and the mechanical resistance to change in direction of movement of the head, i.e., inertia, imposed on the system by the stiffness of the flexible circuit. These factors affect the problem of overshoot and "ringing" or "hunting" in positioning the head at the desired location over the disc, and thus these factors relate to access time. Access time is particularly important in that in some cases access time determines the overall time necessary to complete an application.
Thus, there is a need for a flexible circuit that reduces the amount of energy required to effect flexing, accommodates flexing in a small segment of the circuit, and reduces overshoot and ringing to reduce access time.